The commercial recovery of chrysotile from its ores is typically carried out in a plant in which the asbestos fibre is separated from its host rock by means of a pneumatic system. A complex plant is required, in which a complex sequence of stages of comminution and fibre removal is employed. Despite this, the extraction of the fibre content of some ores is inefficient and a considerable amount of fibre may be left in the tailings from the process. In addition since the longer fibre grades command a premium price, there is frequently considerable loss of product value by the breaking of long fibres into short fibres during the comminution stages.
It is usually necessary to dry the ore feed to the conventional pneumatic process. The drying operation and subsequent process steps are potential sources of release to the environment of chrysotile-containing dust, with its associated health risks. The statutory authorities charged with safeguarding community health in various countries have established maximum allowable concentrations of respirable asbestos fibres in the atmospheres in which people are required to work. These concentrations limits have been made progressively more stringent in recent years and consequently it is becoming more difficult for dry extraction plants to comply with them.
A means of reducing the possibility of emission of asbestos dusts to the atmosphere is to carry out the various operations in an aquoues medium instead of in air. However, it has not proved feasible, hitherto, to devise a wet process having the efficiency, economy and general applicability required for commercial development.
A wet process for separating asbestos from its ores, for example, has been disclosed in U.S. Pat. No. 4,226,672. This process involves the use of an aqueous medium containing a chemical reagent as an aid in the fiberizing step and to improve the quality of the product fibre. In addition to providing a less dusty and hence less hazardous working environment, and an improved yield compared to conventional dry process, the process of U.S. Pat. No. 4,226,672 can provide a fibre of particular reinforcing value in the manufacture of asbestos cement products, such as asbestos cement sheet. This process, however, has higher costs associated with the use of the chemical agent and in the removal of chemical agent from process liquors for recycling in the process.
Thus it is an object of our invention to provide a wet process for recovering asbestos fibres from a wide range of asbestos-bearing materials, for example, crude ore, mill feed, selected streams from conventional processes, tailing, mine wastes and low grade short fibre.